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Ace, WH2T wrote:
"I realized I have to mention that I use an inductive RF coupling - impedance transformer in my homemade transmatch in order to eliminate a direct d-c path from the ladder line to the transceiver input." Lightning is a threat to antennas and to items connected with them. Static charges may build in a time of clear skies, especially ahead of a thunderstorm. Guy-wire insulators often flashover to announce a storm is approaching. Radio towers in broadcast stations get lightning strikes over and over again, but loss of equipment or even air time is minimal. The beacon atop a tower is most exposed, but it is protected by a vertical metal rod attached to the tower near the beacon and extending well above it. Often this rod is an 8-ft Copperweld groundrod bolted to the tower with its tip point skyward. Pits on the rod show it takes hits. Beacon survival shows it does not take hits. The protection works. Guy-wire insulators are often doubled or tripled at the tower attachment points to discourage flashovers here. An air gap is provided across the tower base insulator. In theory, lightning ionizes rhe air in the gap and shunts the charge to ground before it gets into the feeder system. Spacing is usually adjusted to only slightly wider than the gap will breakdown on the transmitted signal. I`ve examined many of these ball gaps and horn gaps and seen no evidence of flashover. To discourage lightning, a turn or two is often made in the conductor feeding the tower. I haven`t seen pts on the balls or horn gaps indicating that the tiny inductance added by a turn or two in the feedwire does any good. A static-drain choke which has a very high impedance at the operating frequency, but has a low d-c resistance, is often connected across the tower`s base insulator on the line side of the base impedance matching unit. It may be placed on the tower side of the matching unit if there is no d-c continuity through the matching unit. All the stations I`ve worked in had an air-core 1:1 coupling transformer in the tower matching unit. Primary and secondary share the same axis but are seoarated by a metal rake which serves as a Faraday screen. It is a picket fence between the coils. The tines of the rake have no electrical connection at one end, but the backside of the rake connects all the tines together and firmly grounds them. The rake allows magnetic coupling between the coils but prohibits electric field coupling between the coils. The rake is a very effective lightning stopper. It is full of pits where lightning has struck. Best regards, Richard Harrison, KB5WZI |
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